Cloud Haskell is a domain-speciﬁc language for developing programs for a distributed computing environment. Implemented as a shallow embedding in Haskell, it provides a message passing communication model, inspired by Erlang, without introducing incompatibility with Haskell’s established shared-memory

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Cloud Haskell is a library for distributed concurrency in Haskell. The purpose is to make it easier to write programs for clusters of machines. It provides a message passing communication model, inspired by and very similar to that of Erlang.

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concurrency.

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== Availability ==

== Availability ==

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Cloud Haskell is available from Hackage as [http://hackage.haskell.org/package/distributed-process distributed-process]. You might also want to install [http://hackage.haskell.org/package/distributed-process-simplelocalnet distributed-process-simplelocalnet]. The cutting edge development version is on [https://github.com/haskell-distributed/distributed-process github].

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Cloud Haskell is available from Hackage as [http://hackage.haskell.org/package/distributed-process distributed-process]. You will probably also want to install a backend:

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* The [http://hackage.haskell.org/package/distributed-process-simplelocalnet distributed-process-simplelocalnet] backend is designed to get you started and experiment with Cloud Haskell on your local machine or local network.

The cutting edge development version of Cloud Haskell is on [https://github.com/haskell-distributed/distributed-process github] and various resources are available via the [http://haskell-distributed.github.com website].

There is also the older prototype implementation [http://hackage.haskell.org/package/remote remote] (also available from [https://github.com/jepst/CloudHaskell github]).

There is also the older prototype implementation [http://hackage.haskell.org/package/remote remote] (also available from [https://github.com/jepst/CloudHaskell github]).

* '''video:''' ''(1hr)'' [http://skillsmatter.com/podcast/home/cloud-haskell/ac-5258 Cloud Haskell]: a general introduction and tutorial, focusing on what it does and how to use it. It also covers some details about the current implementation.

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* '''video:''' ''(1hr)'' [http://skillsmatter.com/podcast/home/haskell-cloud/js-4179 Towards Haskell in the Cloud]: an older but more detailed introduction by Simon Peyton Jones about the problem area and the design decisions and internals of Cloud Haskell. In particular it covers the details of how sending functions over the wire really works.

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* '''video:''' (''25min'') [http://www.youtube.com/watch?v=1jJ2paFuErM Cloud Haskell 2.0] A more technical overview of the new implementation (the [http://www.haskell.org/wikiupload/4/46/Hiw2012-duncan-coutts.pdf slides] are available too).

For an overview of Cloud Haskell it's probably a good idea to read ''Towards Haskell in the Cloud'' (details below). The relevant documentation (in order of importance is)

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For an overview of Cloud Haskell it's probably a good idea to read ''Towards Haskell in the Cloud'' (see just above). The relevant API documentation of the <tt>distributed-process</tt> package (in order of importance) is

We need your help! The [https://github.com/haskell-distributed/distributed-process/issues?state=open issue tracker] on github lists all currently known issues; we also maintain a separate page with the [https://github.com/haskell-distributed/distributed-process/wiki/Important-Open-Issues most important open issues]; the [https://github.com/haskell-distributed/distributed-process/wiki wiki] generally contains more developer oriented documentation, though possibly not enough (the implementation of the network transport layer is documented [https://github.com/haskell-distributed/distributed-process/wiki/Network.Transport in more detail]). Patches are most welcome! (Before you spent serious time on an issue it might be a good idea to add a comment to an issue with what you intend to do.)

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In addition, if you are experimenting with Cloud Haskell and find problems, or even just areas where the documentation is unclear, please open new issues documenting those problems.

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There is also [https://github.com/haskell-distributed/distributed-process-platform an effort underway] to develop an OTP-like platform for Cloud Haskell. In addition to providing many of the features that OTP offers (e.g., generic managed processes, supervision trees, etc) this layer provides an implementation of the ''task layer'' described in the original paper and implemented in the remote package. There is much to do her also, and any help would be much appreciated!

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== Semantics ==

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[https://github.com/haskell-distributed/distributed-process/blob/master/doc/semantics/CloudHaskellSemantics.pdf Cloud Haskell Semantics] (PDF) is an draft document that gives a more precise semantics to messaging in Cloud Haskell. The semantics is based on the [http://dl.acm.org/citation.cfm?id=1863514 Unified Semantics for Future Erlang] paper, but extends it with a notion of "reconnecting" (this is described in detail in the introduction of the document).

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The document also describes some open issues, in particular in relation to ordering of link and monitor notifications relative to regular messages (and messages sent on typed channels). Note however that Cloud Haskell backends that use the TCP network (that is, all backends currently available) do ''not'' suffer from the problems described in that section (essentially because the TCP transport maintains a single TCP connection between Cloud Haskell nodes and orders ''all'' messages sent on that one connection). However, be aware that if you take advantage of this in your code that your code may not work with Cloud Haskell backends that use more esoteric network transports.

== Other Useful Packages ==

== Other Useful Packages ==

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A core concept in Cloud Haskell is that of ''serializable'' values. The <tt>Serialziable</tt> type class combines <tt>Typeable</tt> and <tt>Binary</tt>. <tt>ghc</tt> can automatically derive <tt>Typeable</tt> instances for custom data types, but you need a package to derive <tt>Binary</tt>. There are various packages available that assist with this:

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=== Serializable ===

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A core concept in Cloud Haskell is that of ''serializable'' values. The <tt>Serializable</tt> type class combines <tt>Typeable</tt> and <tt>Binary</tt>. <tt>ghc</tt> can automatically derive <tt>Typeable</tt> instances for custom data types. For <tt>binary-0.6.3.0</tt> and up, <tt>ghc</tt> can also provide a <tt>Binary</tt> instance if you derive <tt>Generic</tt> and add an empty <tt>Binary</tt> instance ([http://hackage.haskell.org/packages/archive/binary/0.6.4.0/doc/html/Data-Binary.html#g:3 see here for an example]).

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For <tt>binary</tt> versions below <tt>0.6.3.0</tt>, you need a package to derive <tt>Binary</tt>. There are various packages available that assist with this:

* [http://hackage.haskell.org/package/binary-generic binary-generic]

* [http://hackage.haskell.org/package/binary-generic binary-generic]

* [http://hackage.haskell.org/package/binary-derive binary-derive]

* [http://hackage.haskell.org/package/binary-derive binary-derive]

* [http://hackage.haskell.org/package/generic-binary generic-binary]

* [http://hackage.haskell.org/package/generic-binary generic-binary]

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* [http://hackage.haskell.org/package/derive derive]

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<tt>binary-generic</tt> and <tt>derive</tt> have been confirmed to work with Cloud Haskell; the status of the other packages is unknown -- YMMV (please feel free to update this wiki page if you have more information).

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== Migration from <tt>remote</tt> ==

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Here are some suggestions that might ease the migration from the Cloud Haskell prototype <tt>remote</tt> to <tt>distributed-process</tt>.

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* The "implicit" type of mkClosure has changed (implicit because mkClosure is a Template Haskell function). In <tt>distributed-process</tt> mkClosure takes a function of type <tt>T1 -> T2</tt> and returns a function of type <tt>T1 -> Closure T2</tt>. In other words, the first argument to your function becomes the closure environment; if you want two items in your closure environment, create a function of type <tt>(T1, T2) -> T3</tt>; if you want none, create a function of type <tt>() -> T1</tt>.

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* <tt>distributed-process</tt> follows the naming conventions in ''Towards Haskell in the Cloud'' rather than in <tt>remote</tt> so the functions that deal with typed channels are called <tt>sendChan</tt>, <tt>receiveChan</tt> and <tt>newChan</tt> instead of <tt>sendChannel</tt>, <tt>receiveChannel</tt> and <tt>newChannel</tt>.

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* <tt>sendChan</tt>, <tt>receiveChan</tt> (and <tt>send</tt>) never fail in <tt>distributed-process</tt> (in <tt>remote</tt> they might throw a <tt>TransmitException</tt>). Instead, if you want to be notified of communication failure, you need to use <tt>monitor</tt> or <tt>link</tt>.

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* The function <tt>forkProcess</tt> in <tt>remote</tt> is called <tt>spawnLocal</tt> in <tt>distributed-process</tt>

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* The <tt>Process</tt> monad is called <tt>Process</tt> in <tt>distributed-process</tt> (rather than <tt>ProcessM</tt>). Similarly, the type <tt>Match</tt> replaces <tt>MatchM</tt> (and is no longer a monad).

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* Initialization is different. See the documentation of of the [http://hackage.haskell.org/packages/archive/distributed-process-simplelocalnet/latest/doc/html/Control-Distributed-Process-Backend-SimpleLocalnet.html Control.Distributed.Process.SimpleLocalnet] to get started (note that the master/slave distinction in SimpleLocalnet is optional and does not need to be used).

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<tt>binary-generic</tt> has been confirmed to work with Cloud Haskell; the status of the other packages is unknown -- YMMV (please feel free to update this wiki page if you have more information).

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* Peer discovery is different. The functions <tt>getPeers</tt> and <tt>nameQuery</tt> are no longer available. The function <tt>findPeers</tt> from <tt>SimpleLocalnet</tt> replaces some, but not all, of the functionality of <tt>getPeers</tt>. You can use <tt>whereisRemoteAsync</tt> to find processes that have been registered by name.

Revision as of 20:09, 14 February 2013

Cloud Haskell is a library for distributed concurrency in Haskell. The purpose is to make it easier to write programs for clusters of machines. It provides a message passing communication model, inspired by and very similar to that of Erlang.

2 Videos and Blog Posts

video:(1hr)Cloud Haskell: a general introduction and tutorial, focusing on what it does and how to use it. It also covers some details about the current implementation.

video:(1hr)Towards Haskell in the Cloud: an older but more detailed introduction by Simon Peyton Jones about the problem area and the design decisions and internals of Cloud Haskell. In particular it covers the details of how sending functions over the wire really works.

video: (25min) Cloud Haskell 2.0 A more technical overview of the new implementation (the slides are available too).

4 Documentation and Support

For an overview of Cloud Haskell it's probably a good idea to read Towards Haskell in the Cloud (see just above). The relevant API documentation of the distributed-process package (in order of importance) is

6 Contributing

We need your help! The issue tracker on github lists all currently known issues; we also maintain a separate page with the most important open issues; the wiki generally contains more developer oriented documentation, though possibly not enough (the implementation of the network transport layer is documented in more detail). Patches are most welcome! (Before you spent serious time on an issue it might be a good idea to add a comment to an issue with what you intend to do.)

In addition, if you are experimenting with Cloud Haskell and find problems, or even just areas where the documentation is unclear, please open new issues documenting those problems.

There is also an effort underway to develop an OTP-like platform for Cloud Haskell. In addition to providing many of the features that OTP offers (e.g., generic managed processes, supervision trees, etc) this layer provides an implementation of the task layer described in the original paper and implemented in the remote package. There is much to do her also, and any help would be much appreciated!

7 Semantics

Cloud Haskell Semantics (PDF) is an draft document that gives a more precise semantics to messaging in Cloud Haskell. The semantics is based on the Unified Semantics for Future Erlang paper, but extends it with a notion of "reconnecting" (this is described in detail in the introduction of the document).

The document also describes some open issues, in particular in relation to ordering of link and monitor notifications relative to regular messages (and messages sent on typed channels). Note however that Cloud Haskell backends that use the TCP network (that is, all backends currently available) do not suffer from the problems described in that section (essentially because the TCP transport maintains a single TCP connection between Cloud Haskell nodes and orders all messages sent on that one connection). However, be aware that if you take advantage of this in your code that your code may not work with Cloud Haskell backends that use more esoteric network transports.

8 Other Useful Packages

8.1 Serializable

A core concept in Cloud Haskell is that of serializable values. The Serializable type class combines Typeable and Binary. ghc can automatically derive Typeable instances for custom data types. For binary-0.6.3.0 and up, ghc can also provide a Binary instance if you derive Generic and add an empty Binary instance (see here for an example).

For binary versions below 0.6.3.0, you need a package to derive Binary. There are various packages available that assist with this:

binary-generic and derive have been confirmed to work with Cloud Haskell; the status of the other packages is unknown -- YMMV (please feel free to update this wiki page if you have more information).

9 Migration from remote

Here are some suggestions that might ease the migration from the Cloud Haskell prototype remote to distributed-process.

The "implicit" type of mkClosure has changed (implicit because mkClosure is a Template Haskell function). In distributed-process mkClosure takes a function of type T1 -> T2 and returns a function of type T1 -> Closure T2. In other words, the first argument to your function becomes the closure environment; if you want two items in your closure environment, create a function of type (T1, T2) -> T3; if you want none, create a function of type () -> T1.

distributed-process follows the naming conventions in Towards Haskell in the Cloud rather than in remote so the functions that deal with typed channels are called sendChan, receiveChan and newChan instead of sendChannel, receiveChannel and newChannel.

sendChan, receiveChan (and send) never fail in distributed-process (in remote they might throw a TransmitException). Instead, if you want to be notified of communication failure, you need to use monitor or link.

The function forkProcess in remote is called spawnLocal in distributed-process

The Process monad is called Process in distributed-process (rather than ProcessM). Similarly, the type Match replaces MatchM (and is no longer a monad).

Initialization is different. See the documentation of of the Control.Distributed.Process.SimpleLocalnet to get started (note that the master/slave distinction in SimpleLocalnet is optional and does not need to be used).

Peer discovery is different. The functions getPeers and nameQuery are no longer available. The function findPeers from SimpleLocalnet replaces some, but not all, of the functionality of getPeers. You can use whereisRemoteAsync to find processes that have been registered by name.